RD-fl134 169 COMPOSITE MATERIALS FOR MAXILLOFACIAL PROSTHESES(U) i/iFRANKLIN RESEARCH CENTER PHILADELPHIA PRH L HELLER ET AL. RUG Si FRC-R-C4842-4 DADi?-77-C-7959

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COMPOSITE MATERIALS FOR MAXILLOFACIAL PROSTHESES

Annual Progress Report

Harold L. HellerRobert A. Erb, Ph.D. DTIC

AUGUST 1981 2818ill SEC T E 0

B... Supported by

UNITED STATES ARMY MEDICAL RESEARCH AND DEVELOPMENT COMMANDFORT DETRICK, FREDERICK MARYLAND 21701

Contract No. DAMD 17-77-C-7059

Franklin Research CenterDivision of The Franklin Institute

20th Street and The ParkwayPhiladelphia, Pennsylvania 19103

Approved for public release; distribution unlimited.

LLJjThe findings in this report are not to be construed as an official

Department of the Army position unless so designated by otherauthorized documents.

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REPORT DOCUMENTATION PBEFORE COMPLETING FORM

I. REPORT NUMBER 2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER

- / 1. 14. TITLE (and Subtitle) S. TYPE OF REPORT & PERIOD COVERED

Composite Materials for Maxillofacial Annual Progress ReportPostses 9 Aug. 1980 - 8 Aug. 1981

Prostheses G. PERFORMING ORG. REPORT NUMBER

A-C4842-47. AUTHORa) S. CONTRACT OR GRANT NUMBER(s)

Harold L. HellerRobert A. Erb, Ph.D. DAMD 17-77-C-7059

9. PuRFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELCMENT. PROJECT. TASK

Franklin Research Center AREA a WORK UNIT NUMBERS

20th St. & Benjamin Franklin Parkway 62775A.

Philadelphia, PA 19103 3SI62775A825.AB.063

It. CONTROLLING OFFICE NAME AND ADORESS 12. REPORT DATE

U.S.Army Medical Research and Development Command August 1981HQDA (SGRD- RMS) Fort Detrick 13. NUMBER OF PAGES

Frederick, Maryland 21701 22 pages14. MONITORING AGENCY NAME & ADDRESS(il dilerent from Controllin Office! IS. SECURITY CLASS. (of this report)

Unclassified

1Sa. DECLASSIFICATION DOWNGRADINGSCHEDULE

I6. DISTRIBUTION STATEMENT (of thie Report)

Aoed faPubli releoub-

17. DISTRIBUTION STATEMENT (o1 the abstract entered In BIock 20 i dilcrert from Report)

IS. SUPPLEMENTARY NOTES

19 KEY WORDS (Continue on reverse aide If necessary and Identify byv block number)

MAXILLOFACIAL PROSTHESES; PROSTHETIC MATERIALS: MICROCAPSULES:

SOFT FILLERS; ELASTOMER COMPOSITES

2,. ABSTRACT (Continue on reverse side It necesary and Identify by block number)

",The purpose of this program is to develop ultrasoft composite materialsto be used as fillers in the fabrication of maxillofacial prostheses.The projected systems are elastomeric-shelled, liquid-filled microcapsules.Improvements were made in the interfacial polymerization process, providingsealed capsules which do not inhibit the cure of matrix materials, andwhich do not harden with time. Further research is needed on larger-scaleprocessing, on making smaller capsules, and on the mechanical propertiesof the ransiles in titrce, . I

DD JAN,3 1473 EDITION OF I NOVSSISOBSOLETE

SECURITY CLASSIFICATION OF THIS PAGE (Wen Data Entered)

I

ABSTRACT

The purpose of this program is to develop ultrasoft composite materials tobe used as fillers in the fabrication of maxillofacial prostheses. Theprojected composite systems are elastomeric-shelled, liquid-filledmicrocapsules. Experiments continued on the interfacial polymerizationprocess, with spherical, sealed, capsules achieved. Diffusion of core liquidthrough the capsule walls has been reduced and the use of a tin catalyst haseliminated the cure inhibition of the matrix materials. Needs identified arebetter production methods, a reduction in capsule size and a catalyzed bathusing a solvent other than kerosene.

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FOREWORD

The concept behind this program is that a multiphase composite systemshould be able to simulate the mechanical properties of human soft tissuebetter than a homogeneous system could. The proposed composite of particularinterest consists of liquid-filled, elastomeric-shelled microcapsules heldtogether to form a deformable mass; this is to simulate the semi-liquidc ellular structure of human soft tissue.

The fourth year's program has been directed toward the elimination ofcure inhibition of the matrix elastomers, the elimination of the diffusion ofcore material through the urethane shells, and to some degree, the reductionof the capsule size. Cure-through over a period of time was a problem, andthis has been eliminated by reducing the skin forming time in the bath to twominutes. Excellent quality, nearly transparent microcapsules can be made insmall batches.

00Frankin Research CenterA iCoan of The FranJh Insu ulet

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CONTENTS

Section Title Page

ABSTRACT

FOREWO RD

1 INTRODUCTION. . . . . 1

2 DEVELOPMENT OF PRACTICAL TECHNIQUES FOR THE PREPARATION OFLIQUID-FILLED POLYURETHANE CAPSULES . . . . . . . 3

2.1 Summary . . . . . . . . . . . . 3.

2.2 Two-Stage Drop Interfacial Polymerization System. . . 4

2.2.1 External (Continuous) Phase . . . . . . 4

2.2.2 Interior (Droplet) Phase . . . .... 4

2.2.3 Interior (Droplet) Phase Formulation . .. . 5

2.2.4 Preparation of Interior (Droplet) Phase . .

2.2.5 Preparation of Polyurethane Capsules by

' - Drop Interfacial Polymerization . . . . 6

2.3 System Variables . . . . . . . . . . 6

2.3.1 Core Materials . . . . . . . . . 6

2.3.2 Batch Age . . . . . . . . . . 8

* 2.3.3 Bath Thickeners . . . • . . . .. . 8

2.3.4 Hicrocapsule Washing . . . . . .. . 9

2.3.5 Catalyzed Bath 10

3 REDUCTION IN MICROCAPSULE SIZE . . . . . . . 11

4 COMPATIBILITY WITH CASTABLE ELASTOMERS . . . . . 12

* : 5 MECHANICAL PROPERTIES . . . . . . . . .. . 14

*. 6 FUTURE PLANS . . . . . . . . . ; • • • 17

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FIGURES

Number Title Page

1 Polypropylene-Glycol Filled Microcapsules (2X) . . . . 7

.; 2 Cut Cross Section of Microcapsules in MDX 4-4210. . * 7

3 Compressive Properties of Urethane Microcapsules WithLiquid Cores . . . . . . . . . . .. . 15

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1. INTRODUCTION

The soft tissues in maxillofacial areas have complex mechanical properties,

and are difficult to replicate when preparing external facial prostheses. An

area in particular in which further improvement is needed in facial

prosthetics is in simulating the softness or "feel" of underlying soft

tissues. This is particularly important if some movement capability is

needed. The softest materials presently available are polymeric foams (which

have the disadvantage of taking a permanent set by loss of gas when

compressed) and gels (which are often unstable and sometimes lose internal

liquid by syneresis).

This program is studying a new class of materials for use in fabricating

maxillofacial prostheses: namely, liquid-filled, elastomeric-shelled

"* microcapsules. Conceptually, such a product is attractive for several

reasons: (1) the cells in the natural soft tissue are themselves composites

* of liquid (or semi-liquid) material in deformable shells; (2) the

liquid-filled microcapsules could be stable entities free from the syneresis

or gas-leakage of other soft materials; (3) the microcapsules could be stored

as such and used by the prosthetist as an ultrasoft filler to modify other

materials as needed.

In the first annual report the history of materials for maxillofacial

prostheses was reviewed. Many materials have been used, but in recent times

poly(vinyl chloride) plastisols, polyurethane compositions and silicones have

been used effectively in simulation of skin and external features.

In the second annual report, efforts toward producing microcapsules by

* two experimental approaches were described. One approach involved coaxial

extrusion of a catalyzed elastomer precursor and core liquid into a receiving

bath. The other approach involved the interfacial polymerization of

polyurethane around droplets of a core liquid suspended in a continuum

containing reactive materials.

At the end of the second year, the coaxial extrusion approach was

discontinued. The third annual report covers the further development of the

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interfacial polymerization process to the point where microcapsules could be

produced.

Work over the past year has included the successful switch to a tin

catalyst to eliminate cure inhibition of the matrix polymers and defining the

variables that affect the quality of the microcapsules.

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2. DEVELOPMENT OF PRACTICAL TECHNIQUES FOR THE PREPARATIONOF LIQUID-FILLED POLYURETHANE CAPSULESS

2.1 SUMMARY

An effective and flexible process has been developed for the preparation

of liquid filled polyurethane capsules of the type believed to be suitable for

use in preparation of polymer composite systems.

The new method utilizes a two-stage polymerization process in which a

fragile polyurea skin is rapidly formed around a liquid droplet by interfacial

* polymerization as the first stage. After initial skin formation, a

* polyurethane wall membrane having the desired physical properties is formed by

a slower, secondary process.

Capsules with strong, flexible wall membranes containing a variety of

.' internal phases have been prepared by this method. The procedure appears to

*" be readily adaptable to scaleup operations.

The urethane system selected for the capsule wall is of the

cycloaliphatic diisocyanate type used to produce low modulus, light stable,

elastomeric films. The liquid interior phase used at present for the

maxillofacial prosthesis application is a non-reactive polypropylene glycol

(Union Carbide Corp., PPG-2000).

Throughout most of this program, the core materials in the microcapsuleshad been found to slowly exude through the urethane walls. This problem had

been overcome by using a higher molecular weight core material and reducing

the curing time in the kerosene bath from over 20 minutes to two minutes.

Another problem solved in this year's work was the inhibition of the cure

of matrix.polymers from the presence of the amine catalyst used in the

isocyanate/polyol reaction. This catalyst has been replaced with a tin

catalyst which does not inhibit the cure of the matrix polymers.

In this report the present capsule forming system and the problems that

have been encountered are described in detail.

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2.2 TWO-STAGE DROP INTERFACIAL POLYMERIZATION SYSTEM

2.2.1 External (Continuous) Phase

Kerosene (Fisher, deodorized, 96 wt. %), 1,7 diaminoheptane (Aldrich

D1740-8, 1 wt %), and fumed silica (Cabot Corp., Cab-O-Sil M-5, 3 wt %) are

mixed with a high speed mixer to form a thickened exterior phase. The silica

thickener is required to control the rate of fall of the liquid droplets

through the exterior phase during the first stage of cure. The silica level

is dependent on the viscosity of the droplet phase. 1,7-diaminoheptane reacts

rapidly with the droplet to form a fragile "skin" which protects the capsule

during the polyurethane formation.

2.2.2 Interior (Droplet) Phase

The interior phase consisted of:

(B-1) a cycloaliphatic diisocyanate(B-2) a mixture of linear and chain-branched polyols capable of reacting

with (B-i) to form a polyurethane(3-3) a tin catalyst(B-4) an inert polar liquid(B-5) a trace of non-reactive dye (eosin) to facilitate identification of

the beads.

B-1 - Isocyanate

The isocyanate presently used is methylene-bis-(4-cyclohexylisocyanate)

(Desmodur W; Mobay). It is used in the manufacture of uon-discoloring

urethane. It is sensitive to water and humid air and must be stored under a

blanket of dry nitrogen. The stannous octoate catalyst (B-3) is added to the

isocyanate.

B-1 - Polyol Mixture

Pluracol P-2010, a Wyandotte diol (98.7 parts) and Pluracol PeP-450, a

Wyandotte triol (1.3 parts) are mixed to form a reactive polyol phase capable

of reaction with the isocyanate (B-1) to form a polyurethane. The polyol

mixture (B-2) is added to the catalyzed isocyanate (B-l).

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1 anklin Research CenterA Dhison o The Fraflu i Ifgsume

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B-4 - Inert Polyol

The inert polyol core material presently used is polypropylene glycol

(PPG-2000; Union Carbide). The eosin dye (B-5) is added to this polyol to

improve the visibility of the microcapsules. The colored core material is

added to the urethane (B-2 + B-1,3).

2.2.3 Interior (Droplet) Phase Formulation

The basic interior phase formulation presently used for microcapsule

formation is as follows:

Isocyanate 99.8[ 72.7Stannous octoate 0.21

50

Pluracol PL-2010 98.7 27.3

Pluracol PeP-450 1.3J

PPG-2000 100.0 50Eosin traceJ

2.2.4 Preparation of Interior (Droplet) Phase

The catalyzed isocyanate is heated to 50*C and agitated by a magnetic

stirrer. The polyol phase is added in a dropwise manner at approximately onedrop per second. Since the isocyanate is sensitive to moisture the entire

, system is blanketed in dry nitrogen. The reaction appears to be nearly

complete within an hour after the addition of the polyols, but the batch

should not be used until the next day. The shelf life of this polyurethane is

about 10 days, but it is best not to use it beyond five days.

The colored liquid core material is added to the polyurethane phase and

thoroughly blended together. Due to the high viscosity of the urethane, air

bubbles are trapped in the mixture and must be removed. This is accomplished

by placing the batch in a vacuum for about 10 minutes. The bubble-free batch

should not be used for several hours, but after this time, it is stable for at

least four days.

19 n::nin Research CenterA Omsmin d The Frarnin institute

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2.2.5 Preparation of Polyurethane Capsules by Drop-Interfacial Polymerization

The freshly mixed external phase (A) is placed in an open vessel

containing a suitably sized polypropylene mesh basket for collection and

isolation of the capsules.

The internal phase is charged to a motor-driven syringe fitted with asuitable needle (a short 22 gauge needle was found to be suitable for many

preparations, but other sizes can be used).

The internal phase is added dropwise to the curing bath with the needle

tip approximately 4-8 cm above the bath surface. A variable speed turntable

• .. may be used to rotate the curing bath to provide a fresh surface for each

droplet.

The droplets immediately form a fragile (polyurea) skin. They are

allowed to remain in the curing bath for 2 to 3 minutes and then removed from

the bath (via the mesh basket). They are rinsed in kerosene followed by a

quick rinse in a dilute nonionic detergent solution (1% Triton X-100, Rohm and

Haas Company). They are then removed from the basket, dried, and stored in a

closed jar.

Figure 1 shows an example of the capsules made by the process mentioned

above. Note that these capsules (shown at 2X) have nearly transparent shells,

and that many have tails, some as long as twice their diameters. These tails

do not present a problem in handling the capsules and should not interfere

with the matrix polymers.

The tendency for tails to form is the result of the viscosity of the

urethane increasing with age and/or dropping the drops from an insufficient

height.

2.3 SYSTEM VARIABLES

2.3.1 Core Materials

The standard core material in the past year had been UCON fluid LB-385, a

polyalkylene glycol. It had been noticed that fresh batches of the prepolymer

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Figure 1. Polypropylene-Glycol Filled Microcapsules (2X)

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and the LB-385 produced poor quality microcapsules, but after standing for

several days, much better quality beads could be made. There appeared to be a

delay in complete mixing which could be seen by the slow development of the

eosin dye in the mix. On the other hand, the LB-385 is miscible with the

kerosene and causes a weak seal at the top of the microcapsule.

In order to overcome the miscibility problems, polypropylene glycol (PPG)

was evaluated as a core material. Using PPG, fresh batches could be used

within several hours, and the weak point at the tail end of the capcules was

eliminated. Although the bleeding out of the core material was greatly

reduced with the two minute time limit in the bath, this problem still

remained on long-term aging with the LB-385. A series of PPG liquids was

evaluated using a range of moleculat weights from 425 to 2000. The degree of

bleeding correlated well with the PPG molecular weight. The MW limit of 2000

gave the best results, but due to its high viscosity, larger drops are

formed. Also, the rate of flow out of the needle must be carefully

controlled, since higher rates produce an unbroken strean of the urethane

going into the bath.

2.3.2 Batch Age

The prepared urethane batches are not stable and start to gel in about 7

days. Up until this time, they can be successfully used. There is a shorter

shelf life for the urethane after the core material is added. Gelling does

eventually occur, but the quality of the microcapsules is reduced by the

seventh day of shelf life. As mentioned earlier, fresh batches need to be

aged several hours before using.

2.3.3 Bath Thickeners

The kerosene bath is thickened with fumed silica to prevent the

microcapsules from hitting the bottom of the container when they are dropped

into it. HiSil 600 )iad been used in the past, but Cab-O-Sil M-5 is now used.

The M-5 is clearer and disperses more readily in the solvent.

1' rnkfin Research Center -8-A Oiluo of The Frarnkho Insit4@

Now that a higher molecular weight core material is used, the droplets

must be released from a greater distance above the bath to allow the tails to

contract before passing through the surface. Four to eight centimeters above

the bath is about ideal, and three percent Cab-O-Sil is about the ideal

concentration. Higher concentrations were tried to eliminate the tails, but

this caused irregular shell surfaces that had thin spots. The three percent

level allows for some settling so that the drop initially comes in contact

with the pure solution, forms a skin, and then comes to rest in the Cab-O-Sil

layer.

2.3.4 Microcapsule Washing

After the microcapsules are removed from the bath, they must be washed,

chiefly to get rid of the kerosene. They are first rinsed in clean keroseneto remove any Cab-O-Sil and remaining diaminoheptane. From this point,

different methods have been used. The earlier approach was to quickly washthe microcapsules in an aqueous detergent solution followed by drying them on

a paper towel.

Alcohol washing has been found to be unsatisfactory due to softening of

the shells. ThL present technique used is to store the microcapsules in

polypropylene glycol (PPG-425) after the kerosene rinse. The microcapsules

are dumped out of the basket onto a paper towel to drain off excess kerosene,

-" and then placed in the PPG for storage until the production run is finished.

Storage in the PPG allows further curing without sticking together. This bathalso removes any stray diaminoheptane by chemically reacting with it, forming

a solid that is easily removed.

An interesting approach was tried for a continuous production technique

which utilized a two layer bath. The lower layer consisted of PPG and the

upper layer of slightly thickened kerosene containing diaminoheptane, and

. having a depth great enough to provide sufficient cure as the microcapsules

*. pass through it. Unfortunately, the diaminoheptane gradually reacted with the

PPG and formed a barrier at the interface.

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The microcapsules can remain in the PPG for one day, but then should beremoved and washed with water and dried for future use. Other solvents

evaluated for washing include toluene and petroleum ether. Toluene issatisfactory and eliminates the need for a water wash. Petroleum ether, at

this time, appears to be an excellent solvent wash.

23.5 Catalyzed Bath

The diaminoheptane appears to have a limited solubility in the kerosene,

but up until recently, no problems were encountered from this. Recently itbecame time to produce larger batches of the microcapsules for composite

testing, but it was found that the larger the single batches were produced,the poorer the quality of the microcapsules became, particularly in the shell

elasticity.

This indicates that an insufficient amount of catalyst is available for a

complete cure when a large number of droplets are released in the bath. Thissuggests the need for a solvent other than kerosene which itself has in the

past been a problem.

Preliminary tests show that petroleum ether readily dissolves the

diaminoheptane, thickens well with Cab-O-Sil and produces tough smallerdiameter capsules than those formed in kerosene. The cure rate in thepetroleum ether is so rapid, it is possible to remove the microcapsules from

* the bath within a half minute.

Petroleum ether evaporates rapidly and may cause some handling problems,

which need to be considered.

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3. REDUCTION IN MICROCAPSULE SIZE

By the present method of production, the capsule size is held nearly

constant by the viscosity and capillarity effects that come into play when a

drop falls free from the tip of the needle. The needle size has only a minor

effect on the capsule size which is generally two millimeters in diameter.

Two air systems were tried for reducing the drop size. The first system

was designed to produce jets of air around the flat tip of the needle to blowoff small drops. The air was directed at the tip in a steep angle to force

the drops away from the tip. This process did not blow off the liquid in

small droplets, but removed the liquid in a spray mist.

The second approach tried was to inject the air into the liquid streamback inside the needle, but this too only removed the liquid in a spray mist.

From observing these processes, it was believed that the liquid drop mustbe completely out of the tube before it could be knocked away. Thus, by

extending a tip down one side of the needle, it was thought that it might bepossible to blow some of the liquid free from the free hanging drops. Again,

this approach did not work.

One other approach tried was to disperse the urethane into small droplets

in a liquid in which the urethane and core material are not soluble, and thendumping this mixture into the diaminoheptane bath. This approach has shown

some success using silicone oil (Dow Corning 200) as the dispersing liquid.

For a given batch, microcapsules in various sizes ranging from 0.3 to 1.5 mm

in diameter were produced. These microcapsules cured through to solid beadsin several days. It is possible that this method can be further improved, but

at this time the variables affecting the quality of the microcapsules are bestobserved and controlled by the single drop production method.

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4. COMPATIBILITY WITH CASTABLE ELASTOMERS

Up until this year, the microcapsules could not be used in bulk with

castable elastomers due to the cure inhibition of the matrix elastomers by the

amine curing agent used in forming the polyurethane. To avoid this problem we

have switched to tin catalysts. Dimethyltin was found to be unsatisfactorydue to its incompatiblity with the diaminoheptane. Stannous octoate was found

to be ideal. In small quantities (0.2%) it provides a fast method forproducing urethanes that do not inhibit the cure of the microcapsule shells.

Matrix elastomers of Dow Corning MDX 4-4210 Clean Grade Elastomer,*! Silastic 382 Medical Grade Elastomer and RTV 3145 can now be used without

inhibition from the urethane catalyst. Figure 2 shows a cut cross section ofcapsules imbedded in MDX 4-4210.

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5. MECHANICAL PROPERTIES

An improved testing device was built for measuring the mechanical

properties of single capsules and capsule composites. The new device utilizes

a piezoelectric crystal for measuring the load applied to a bead. This device

has the advantage of being much more sensitive than the strain gauge system

previously used. This system also as the advantage of an internal constant

-: calibration, and it uses a non-bending platen. Furthermore, the platen is

large enough to test cubes of capsule filled composite materials. The strain

.* gauge arrangement previously used was operated at its maximum output which

yielded a full scale readout of 400 grams. The piezoelectric device is used

at an output level well under its full capacity. It was found that a full

scale reading of 100 grams is about ideal for our present capsules.

- The need for a better measurement technique resulted from our

requirements to measure small changes in the compressive strength of the

microcapsules as they age.

We are now able to calculate the forces applied to a single spherical

capsule. All previous results were reported only as the applied load. The

problems encountered with measuring the force per unit area was that the area

of the contact surfaces of the capsules increased with increasing load. To

overcome this problem, Dr. J. Stuart of Franklin Research Center (FRC) derived

an equation for determining the radius of the contact surface for elastic

spheres under compression. This equation is given below.

B "(1.3R _ O.013d2) 1/2 0.393d

where B is the radius of the contact surface, and:

R - the original radius of the sphere before compressiond - the distance between the compression plates.

An example of the results obtained by using the new compression rig and

calculating the applied forces using the above equation is shown in Figure 3.

The results obtained in this figure are averaged from five microcapsules taken

from batch 149-9.

-13-UOUUkra'kin Research Center

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250

200

cc

150

1000

50

0 5 10 15 20 25 30 35 40' 45 50 55 60

DEFLECTION (percent)

Figure 3. Compressive Properties of Urethane MicrocapsulesWith Liquid Cores

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VVtld~kin Research CenterA D-m of Th* Fuanhin Insolute

Microcapsules from this batch have been tested over a two month period,

starting with microcapsules only several hours old. Over this time period

there appears to be no perceptible changes in softness and elasticity of the

urethane shells.

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6. FUTURE PLANS

The major problem encountered a year ago was the izability to define the

variables that had an adverse effect on capsule quality. Now that these

variables are much better understood and can be controlled, future work should

advance significantly.

Four areas in which further work needs to be done are as follows:

1. Reduction in Microcapsule Size

To be used as a practical filler system, the diameter of the capsules

should be substantially less than the thickness of the 7rosthetic structure to

be fabricated, the present capsules, at about 2 mm in iiameter, are too large

for most applications. Efforts will be made to prepare capsules in diameters

of 0.5mm and less. Approaches which will be considered are: using smaller

diameter needles (than the present 22 gauge) for droplet formation, employing

vibration techniques to produce smaller droplets, and disgersions in neutral

liquids.

2. Advanced Production Methods

The present production method is ideal for ulaking emperimental batches,but not efficient enough to produce large quantities of 13he beads. We have

not been working in this area, since the approach to be Uaken will depend

somewhat on the method finally adopted for making the mxrocapsules. Three

possible approaches include (1) multiple containers on a rotating table, (2)

removal from the bath by a continuous belt, and (3) coniinuous bottom draining

through a filter and recirculation of the liquid back iVD the top of the bath.

3. Improved Diaminoheptane Bath

The use of kerosene as the liquid portion of the b~ch has presented some

problems with the rate of production and the permeabili, of the capsule

shells. Other bath liquids such as petroleum ether will be evaluated.

1 FnWIn Research Center -16-A Oom 0 The Fiandn Ilsauite

ANN -.

4. Composites

It is now possible to produce composites without cure inhibition of thematrix polymer. The limiting factor now is the present production rate. With

increased production, various composites will be made and mechanically tested.

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I copy -Deadco School of Medicine

Uniformed Services University of theHealth Sciences4301 Jones Bridge RoadBethesda, Maryland 20014

I copy SuperintendentAcademy of Health Sciences, US ArmyATTN: AHS-COMFort Sam Houston, Texas 73234

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